Extrapolating pharmacological and surgical therapies from adult (AD) studies to infant (INF) patients is problematic because the knowledge of cellular electrophysiology and molecular biology of human INF heart cells is limited. We have studied developmental differences in rabbit ventricular cells and now extend these studies to atrial and ventricular cells isolated from AD, young adult (YAD) or INF patients. SA 1: Developmental differences in transient outward current of atrial cells. We will extend our studies to isolated cells and tissue from YADs (age 14-20). In addition, several other accessory beta-subunits have been found in cardiac myocytes and may interact with Kv channels and regulate the function of these channels. We will determine relative amounts of these putative regulators of human atrial Ito to determine which correlate with developmental changes in Ito kinetics.SA2: Developmental differences in amplitude and regulation of calcium current in atrial cells. We hypothesize that INF atrial cells have tonic inhibition of adenylyl cyclase (and thus of ICa) mediated by inhibitory G proteins, possibly related to constitutive activity of the adenosine A1 receptor, and that, compared to AD or YAD cells, have greater sensitivity to inhibitors of phosphatases and phosphodiesterases, and that developmental changes in basal ICa amplitude and beta-sympathetic modulation correlate with inhibitory G protein levels, receptor numbers for M2 and A1 receptors, and constitutive inhibitory activity. SA3: Modulation of atrial cell calcium transients by changes in AP waveform and developmental age. We will test the hypothesis that prolongation of the early repolarization phase of the AP increases Ca2+ entry and that YAD cells have faster removal of Ca2+ from cytoplasm than INF cells and we will determine if the Na- Ca2+ exchange current (INCX) is greater in INF vs. AD or YAD cells. SA4: Developmental differences in Ca current and transients in ventricular cells. We propose that INF cells and tissue have lower basal ICa, lower potency for ISO stimulation, higher levels of Gia3 and A1 receptors, greater inhibitory potency for adenosine, and tonic inhibition of ICa. We also propose that the YAD cells have lower levels of NCX and lower INCX, higher levels of SERCA and faster removal of Ca2+ from the cytoplasm. Previous animal studies have indicated various developmental changes in cardiac cells. We will specifically study human postnatal developmental changes in Ito regulation of ICa and intracellular Ca2+ transients.